I would say infantry would work together mounted troops would not, mind you I am not the kind to be easily bothered by scale discrepancy.
I agree with what others say that the casting on steel fist is considerably better than on their Perry counterparts. Yes they are more expensive but the extra price buys a great deal of quality.
What are you looking to play? If its something small like Ronin might be worth going full steel fist. Especially with the new Teppo and Yari Ashigaru coming out off the back of the kickstarer. If however you are looking for a larger force and want to mitigate some of the costs I would recommend using perry for your rank and file and steel fist for your unit leaders. The extra size of the steel fist would make them stand out as leaders.
Anyway onto the size comparisons and you can decide for yourself.
Guy with the big red hoop is steel fist, The rest are perry

Cavlary, Perry on the right and steel fist is the one next to it

Steel fist in the very middle and the unpainted guys either side. Clearly chunkier but to be not so much that I wouldn't use them in the same force.
