If you want to simplify the skills in RMSS you can always just do away with the category and increase the progression of the individual skills from 3/2/1/1 to 5/3/1/1. It'll have the effect of gaining quicker in individual skills, but it wouldn't dramatically change balance because you wouldn't be getting broader category bonuses anymore. Just take the stat bonuses from applying to the category and move them to the individual skills. That'll take a little work, but not much.
Essentially RMU just broke that up into categories then implemented a similar skill type rule, basically taking the RM2 way, but then stacking a new mechanic on top of that for similar skills. In my opinion all that did was complicate the base system... I think it should have been left alone or do what has been done, but without the similar skill rule, that should have been a later add-on and not included in the core rulebook. Categories essentially are a similar skill system when you think about it. You buy into the category and all the related skills get a smaller boost than individual purchasing provides. For some reason a lot of people don't understand that. It's doing what a good number of RM2 users did by implementing a similar skill rule. Although, I can understand those who didn't want to use a similar skill rule not liking it.
So with the RMSS/SM:P thing I guess it depends on what you're trying to do. Getting rid of categories might help with whatever it is. I've never tried to combine the skills of the two. Even in situations where we have a character using both systems we'd essentially just create two separate skill sets for the character.
On the combat side, RMU B2 is better than B1 (which was even more complex), but I agree it adds complexity without adding any real substance. There's a simplicity in having a Snap, Normal, and Deliberate without having to worry about your percentage of activity. It simulated how hastily you did something rather than how much time it took to do it. Obviously with RMU someone was more concerned with trying to measure linear time rather than the slightly more abstracted RMSS round. Trying to meter it by actual time raises more issues than it solves and complicates it at the same time imo. Using the RMSS round, if you have a terrible initiative, you can try to make a snap action at a set penalty without having to monkey around with action points or percentages. RMU kind of does the same thing, but in a slightly more complex manner. It's not a huge change in complexity, but rather just another small shift that stacks up over time.