This might seem like a radical change, but bear with me. What I'm suggesting here is getting rid of credits and points entirely, and simply allowing for you to buy or morph into whatever your team has unlocked. The idea would be to balance all of the choices in a way that allows you to have a variety of options and allows you to experiment, while at the same time not punishing you severely for picking something you found interesting and wanted to try.
A few other games have examples of this. If you look at Urban Terror, you can spawn with any combination of weapons and equipment that you want to, but you're limited to only being able to pick your equipment at spawn time. People use all sorts of weapons, from assault rifles to machine guns and even a grenade launcher, but the game doesn't feel particularly imbalanced. Or, look at the variety of death match games out there, where you just pick up weapons as you jump across the map. If you know where a particular weapon spawns on the map, then it's functionally equivalent to the idea of not having to buy it. You can have competitive balance without limiting someone's choices.
Humans are already limited by having to return to base to restock on ammunition and heal. Aliens can only morph in one direction, but they can probably get some sort of delay between switching forms, during which they're temporarily vulnerable to attack. I don't think that what I'm proposing would necessarily lead into huge packs of battlesuits with lucifer cannons or roaming gangs of tyrants, but if it did, then I would suggest that the rate of momentum decay should increase heavily as it reaches the end of the bar. That way, you have to keep killing opponents if you want to keep access to the good stuff. Once the momentum bar goes down, you can't buy that thing or morph into that form until it fills back up again.
What do you all think? It probably sounds ridiculous from a Tremulous standpoint, but I'd really like some feedback here. One of the most irritating parts of Tremulous, to a new player, was losing the cool thing you got just after getting it. This would slightly tip the game over to having a more casual feel.