Kinetik in back, batcap in front? or run all kinetik?

should be fine

Best would be to have all the same make and model of batts. When you have multiple batts they all act as one. So it doesn't matter which type is up front, all will be used to start the car. When you mix and match batts you could have slightly different voltages which will cause damage to them over time.

I removed my front batt and have 2 group 31's in the back with no problems.

Depending on which 2500 D amplifier you have I would also do dual 1400's in back. Would also depend on your alternator, as with just about all agm batteries if you want them to last a long time when you want them to rest at inbetween 12.7 to 12.8 volts when the car is off and has been off for more than a few hours so you can measure them with no surface charge being left in them from the alternator. Also I do not recommend a tiny starter battery, small 35 to 20 Ah batteries have less capacity which makes them far more subjectable to going bad from parasitic drains and or being left in a slow downward spiral of losing charge over use.

Batcap/