Am I likely to lose more of one particular type of unit e.g. will I lose more of my weaker troops than my stronger troops in a particular battle?

Normally when troops fall in battle the same proportion is lost throughout each group of troops regardless of strength, so whether you had ten cavalry and 500 light infantry or vice versa there would be 50 times more losses in the group that had 500 units. If you correctly estimated that you would lose a quarter of all your troops in a particular Persian position the same proportion would apply to each group whether you had 10, 100 or 1000 of one type of troop e.g. if you had 10 thureos and you lost a quarter you would lose 3 (2.5 rounded up to 3), if you had 100 peltasts you would lose 25 and if you had 1000 javelineers you would lose 250. Veterans count as separate groups so if you had 10 thorakites and 10 veteran thorakites you would lose 3 of each vet and non-vet group so lose a total of 6 thorakites rather than a quarter of the total 20 thorakites which would have been 5 if they were counted as one group. If a percentage of troops lost result in a figure that includes less than 0.5 soldiers then it is rounded down e.g. if you lost a third of ten soldiers you would lose 3 as one third is 3.33, less than 3.5, otherwise if it were 3.5. or 3.6 it would be rounded up to 4. The losses are in proportion whether they are huge or tiny. Occasionally as a result of several being rounded down or up an extra soldier is sometimes added or deducted from the total losses. Troops lost are in proportion whether they fall in Persian positions, cities, emporia, capitals or pantheons.

How many points should I expect to win for each fallen unit?

The return in points is much the same in any PVP whether troops fall in cities, pantheons, capitals or emporia. (The return in points tends to be much less with Persian positions and points won in the small daily coalition missions though this can be made up for if there is a simultaneous PVP tournament where one can earn more points or sometimes the low return in points in Persian missions can be compensated for if you win lots of troops, resources or armour in a particular position.) How many points should you expect? In my experience, by the time you are well past level 10 in your treaties you should be winning on average (for defence troops in PVP) 10 points per phalanx, 15 points per horse, 4 points per heavy soldier and 1 point per 3 light infantry units. By the time you are over level 15 you tend to earn slightly more points unless your opponent has much stronger or numerous troops. By the time you are over level 20 of your treaties you can expect to win up to 2.5 times more points than you were winning when your treaties were under level 15. Occasionally you may win only half what you expected because your opponent is the same level or higher in his/her treaties and has activated all boosts and relevant elixirs and dominion. Political attacks rob a player of about 80% of their normal points although it costs the user of a political attack in drachmas. Fortunately political attacks can't be used on pantheons or I expect we would see a huge exodus from the game.

Sometimes, of course the unexpected happens, perhaps due to a glitch, and a few players receive less than 10% of the expected points in a PVP tournament even taking into account that the other player might be stronger. This is just not possible by normal means. Even a political attack generates more points. I have a long experience of the game and know what points to expect. The way to investigate this if you are an advanced player and know what points you should be receiving is to visit your opponent's city, see what level they are to estimate how far they are likely to be in their treaties and look at their general to see if their general is much better equipped than yours and finally to take into account which of your elixirs are applied and what level they are and whether your dominion was activated and whether your opponent had their dominion activated. Most offensive or defensive boosts that can be won in tournaments or bought with GPs add 10% strength. Some boosts available in packs are 20%. And there is a 50% boost that costs a lot of real money. So if you consider that your opponent could be 20% stronger because s/he is 20 levels higher and probably ahead of you in their treaties then also look at their general and see that the armour makes them another 30% stronger and consider that they may have activated the expensive 50% boost and that they also have 10% dominion activated and their elixirs might be 5% ahead of yours then you could expect your opponent to be 115% stronger than you and win just over twice the points of a player of the same strength as yourself, which means you would win slightly under half the points that you would normally expect. So questions should be asked if you lose troops worth 120 points to an equally advanced player or worth 300 points against a less experienced player or at minimum 60 against the strongest possible player with every boost activated but you only get 9 points.  Questions should also be asked if much greater proportions of your more valuable troops are falling even when there are less of them and proportionally less of them should fall than your weaker troops if your weaker troops are much more numerous.