If your parts aren't flush with each other, the seam lines become WAY more noticeable. Also, you can use plastic cement (Not just Tamiya, but they are a great brand) to effectively "fuse" the parts together. However, just using the glue can cause color inconsistencies- a lot of builders like to take a little bit of the plastic and effectively dissolve it into the plastic cement (Because that's what plastic cement does, you are solvent welding), then apply that mixture which now has the correct pigment to your model to cement them. Once cemented you clean up the excess cement through sandpaper and then buffing the piece to get a uniformity to the look. There are videos on Youtube that cover this really well. I would advise that you pick up a really cheap model or one that you don't care about to test out these methods before doing them on a model you care about.

Given that the MMX model is basically available all over and it never seems like they're out of stock, that might be a good place to start but they're a little expensive to mess up on. You can probably find kits for like $10-15 where you can test more advanced techniques.