In theory it's possible to merge any mods, but if both try to change the same thing, then obviously you'll have to pick one or the other (for example, if both mods try to change the price of capital ships to a different value, can't have both).
Sins is fairly modular in design, so anything new will usually have its own files, so those shouldn't conflict and merging would be as easy as just combining the files. However, everything new added also usually requires modifications to something existing, and that's where the problem comes in.
The easiest and safest way to go about merging would be to get a freeware program that can compare file contents, there are lots of those around. The one I use I forget the name of (will check when I can), but it can basically compare two folders at once, show the differences between pairs of files in them, and is able to write the data from one to the other with a single click. It's very handy. But even with it, some files in Sins rely on the proper counters to work properly. For example, there's a counter for the number of research topics for each race. If one mod adds 3 topics, its counter will be I think 104 (TEC has 101 by default if memory serves). If another adds 5, its counter will be 106. If you merge them, you'll have an extra 9 topics so your counter will need to be 110, but the comparison program doesn't add them, it only replaces, so you'd have to keep track of these counters yourself.
Basically, merging mods isn't too bad if you're familiar with how Sins is set up, but if you aren't you'll likely cause some errors and crashes
Especially in the case of Sins Plus and the SW Mod, both of which are pretty major.
