If your looking at going with a mid travel kit, our MTS kits are designed around using the stock length shocks. So any shock that is the same length as stock (or works on the stock a-arms, as some can be a little longer as the factory likes to be very safe on travel numbers) will physically bolt on to our kit, and work. But, for the best performance, they should be tuned to our kit for the extra leverge the longer a-arms put on the shocks. But, it also comes down to personal preference. If you like a little softer ride than average, then they might not need to be changed. If the shocks are already a little stiff for stock a-arms, they might work great on our MTS kit. But its subjective. If you get Fox shocks, you can always send them to Fox, and have them set up to our valving and spring rates just like you would get if you ordered the shocks specifically for our kit. In that case, depending on how cheap you got the used shocks, you might be able to save a little money. But keep in mind, Fox will charge you to rebuild them to the different spec. Plus, there might be extra money involved for fixing other parts of the shocks that may be damaged that you can't see from the outside.
So to answer your question, there are differences for shocks for our MTS kit, and stock a-arms, but it is just the rate of the springs, and the amounts of valving on the inside.. all of which can be changed. Just so you have an idea, coil springs are around $75 each, and you would most likely need 4 of them, plus whatever the shock company charges to re-valve the shocks. So unless your getting a smoking deal on the shocks, it might be better to just get new shocks.
Not sure what car you have, but with our Rhino MTS kit, and the bilstein shocks we use, the only thing that is different is the spring rate, so the valving wouldn't need to be changed if you wanted to switch them over. And they work really good on the stock a-arms, and really good on our a-arms once the springs are changed.
