OMG this is why mecha people need to post in this forum.
People dont know the crap we go through to get our kits like yours.
There are a few ways of fixing the anus like tightness of the snap kits.
1) You take a file to the peg, and sand it down. You dont sand it enough that it is gone, just make it smaller.
2) You take an exacto blade, stick it in the female hole (brutal imagry...) and twist. It carves the hole larger, a few twists and its perfect. The only issue is some female holes are either to large for the exacto to twist properly, because of the shape, OR the sides of the hole are to thin, and it would destoy it. If either is the case, just sand down the peg.
Doing either of these will allow it to slide into place, creating those almost perfect evennesses we have come to love in scifi kits between pieces. Itl also allow for easy construction and deconstruction, to learn what order you will paint.
If I was better at my craft then I am, I would do a step by step from the very beinning (like, this early of a stage) untill finishing, using AM parts, and metal upgrades, of how to build a gunpla, and send it to fsm