1: Any recommendations on these type of detectors (fixed, rate of rise, etc)?
I'm not sure you can even get heat detectors without rate of rise anymore, but get units with both.
2: Does the same recommendation/requirement apply of using the same manufacturer as the other smokes? (which would make sense since they will all be wired on the same zone, right?)
As upstatemike mentions, heats are very simple (all mechanical, no electronics) and brand doesn't matter. However, IIRC you shouldn't put heats and smokes on the same circuit, as heats are not "life safety"devices as defined in the code. If my intelligence gets back up into the triple digits later today (just got off a 24 hour duty shift, the brain MIGHT be firing on 2 out of 12 cylinders if I'm lucky), I'll dig up some paperwork to check.
My building inspector recommended (it wasn't required) a combination smoke-CO detector for the kitchen, if you cook with gas. However, he stated that a smoke-detector is best on the ceiling, while a CO-detector is best half-way up the wall (I don't know why).
The inspector is right about the mounting locations, but wrong about the smoke detector in the kitchen. In fact, I'm pretty sure the code specifically recommends to AVOID putting one in the kitchen, along with high humidity locations such as bathrooms. CO detectors should be mounted around chest level (actually it's "mouth" level, but who's mouth do you use? Shaq's? Mini-me's?), while smokes go on the ceiling or high on the wall. There are also some limitation with smokes about corners, ceiling ridges, and the top 6 inches on walls (don't for all of them).
I recently learned that a CO detector will not detect propane, unless the propane is burning.
Correct. CO is created during combustion (and political speeches), so putting a CO detector near gas storage is a waste. CO detectors should be at chest height, preferably near the sleeping areas. You can get LP gas detectors, but I wouldn't bother for garage storage of spare bottles. I have LP detectors near the propane furnace, gas cooktop, and gas fireplace. You can also get a natural gas version. IIRC (see brain comments above), the natural gas versions are mounted near the ceiling, while LP is mounted near the floor.