For me, only special warriors such as commanders or player characters are elligible to receive customized units. I would allow for self-designed units as part of a RPG campaign that focusses on the industrial R&D side of things or empire building. Infantry units on the other hand are beast of their own, however, for me as well. I like to design them and give them certain background stories.
Unfortunately, creation rules as presented in the TM make infantry units prohibitively expensive. Take a platoon of 28 soldiers, give them a FC armor set, heavy support lasers and Mauser laser rifles as well as anti-'Mech training, and they cost more than two Stingers, though at roughly halve the BV value.
Now, the in-game value of the meta-game currency "C-Bill" is very much debatable in the context of armament projects - in fact, my theory is, it's completely useless - but it's all we got. FASAnomics do not belong here, however.