Yes.

The polygon limit is...  not exactly fixed, but models of more than a certain complexity will crash the conversion tools or the game.  <1500 polies is generally considered safe.

I heard there was a handy new video tutorial for SS2 mesh converting around here, you might find it helpful.

If i make a weapon model,then do I have also have to make a pair of hands?Or can I use those that are ingame?

The important thing with regards to the polygon limit is to stay as low as possible but don't concern yourself with the number of polygons as much as the "quality" of the polygons.  Try to avoid intersecting geometry (cutting into planes), minimize geometry that isn't closed (ie. missing polygons on the surface), keep the vertex and edge angles relatively large so that you don't have really skinny "sliver" triangles or wedge-like edges.  These tend to crash the importer.  There are usually multiple ways to arrange a bunch of triangles within a given face and some configurations give you more well behaved triangles than others so don't be afraid to turn a few edges to see if it helps make the triangles more chunky looking as opposed to "sliver" looking.

Thanks.  I guess when you don't know how to do something properly (modelling) and you're too lazy to get professional instruction, you tend to do a lot of things you're not supposed to do as you experiment because you don't know any better.  In my ignorance I stumbled upon a few interesting things.