What I did with my chapter background, which may be applicable to you, is do away with any idea of progression between squad types. Marines are marines, and if you're good enough to become one you're already a master of war. Battle companies have 100 dudes (and dudettes ) and what they do [URL="http://collegiatitanica.blogspot.com.au/2009/07/rainbow-warriors-chapter-variance.html"]depends on the tactical situation needed[/URL]. My 1st company isn't vets but specialists - recon teams, sappers and so on - so if a line trooper is noted as exceptional in one area they get moved there.

Could work in your situation so you can have them running around with lascannons and such one day then strapping on jump-packs the next without worrying about some concept of progression. Also means you can ignore a squad type if nobody likes it.