I currently have 256gb SSD on my iMac and then I have external HDDs which hold my music, video and photo libraries.
Being HHDs they are obviously slower than you will have if you get an external SSDs.
I've had that setup for a while and it works fine. However I do get fed up with the wait sometimes when using them. Part is the wake up time if, for example, I start playing music after the drive hasn't been accessed for a while, which again shouldn't be as bad for you if using SSD's (although I suspect there will still be a wait sometimes and being external will be slower than the internal).
I have decided to go with the 2TB version this time so I can get my music and photo libraries back internal (my TV/Movie library is about 5TB so that has to stay external). I'm really looking forward to the extra speed when starting music or loading photos) but I really don't think that you will have a problem with the setup you suggest.
One other thing, whilst making my decision I did look at various options for external SSD's and was surprised at the number of reviews I read from people complaining that they were slow. That seemed to be for many different models. I am always reluctant to read too much into reviews from strangers, but it was a factor in my final decision.