The normal advice about maintaining some consistency in weight or speed can work, or you can build the lance with those differences in a way that still functions effectively as a team. Going with 4 slow mediums or low-end heavies is fine for holding a solid line, but using long-range snipers and a light, fast spotter can also work. In some situations, three fast scouts and a heavier "bodyguard" that they can fall back on for protection is also viable. Then you have "cavalry" lances, where everything is at least high-side-of-average speed, with enough weapons and armor to engage other medium or low-end heavy opponents, yet fast enough to pull back if outmatched.
Ultimately, go with whatever works for you, at least until you've got enough experience and skill at the game to utilize other combinations to good effect.
As for the combination you mentioned, you can either use the ranged firepower of the -A-6 to sit back and support whatever's in your front line, or use the -A-1 as part of that line. The rest of your front line and what kind of opposition you expect to face should determine your answer. I'd be tempted to use the -A-6 to support the sturdy T-bolt and whatever else I've got from a safe distance behind, where its armor isn't as critical. The -A-1 would be preferable in a combined arms engagement where I'm expecting to face armor. Tanks HATE SRM spam, although an SRM-2 is hardly frightening.
I'd also tend to fire the PPCs when I get good odds with the -A-6, and fire the LRMs when the odds are questionable and I don't want to build heat. If the odds are really good, like 12 hexes (Medium Range for the PPC and LRMs) with only 0 or +1 total movement and terrain modifiers, and I'm not already running hot, I'd consider firing everything. Don't turn down a gift when it stares you in the face, but don't build heat if it's not worth the potential loss of an even better shot next round.