Dragonman said:

how do i get the legeuary units

You buy them, but they are offered only around and shortly after New Year.

Dragonman said:

how can i get better ships like the ussr and the alabma destroyer

These two do not exist in this game.

Your troop strength tiers are as follows.

Armada are top level troops.

Fleet are second class units.

Mercenary 3rd rank soldiers. (should always be on the production line like pirates)

Pirates dead last cannon fodder. (build a ton of them)

DREAD UNITS are the support staff for the above army.  They can be purchased in your smuggler's den.  As Crowbar mentioned around New Year's the LEGENDARY DREADS........Icebreaker (offensive) and Leviathan (defensive) usually become available for purchase.  They generally cost the same as a Plague Ship or Kraken.  They have the advantage of pumping both Fleet or Armada should you lack enough of the regular dread required for this task.  There is one small advantage they can give you but that is a different topic.  Regular dreads will work just fine too.

It is important to keep all your troop ques producing around the clock.  IMO there is never an excuse not to be producing the lower 2 tiers.

  The most common way to build up big Fleet and Armada class units is generally not thru production but rather sacrificing the lower two ranks in your Lighthouse Prizes to "upgrade them".  There is a whole thread on Prizes if you want to learn more about "banking" and other strategies for this.  Timing is also important.  Build up and take advantage of "prize tournaments".  Until then just produce units and take out some of your smaller prizes level 20 or less daily to build up your "bank" and give you higher prizes to hit during the tournament.

In the game we shall refer to your ussr as a "submarine" and your alabma slamma as a "Juggernaut" aka Sub and Jugg and they would be Armada level units.