Technically it's possible to place any vanilla weapon on a spidertron, but there's good reasons not to. First of all, vanilla shotguns and tank cannons have friendly fire, and Spidertrons with those weapons tend to blow themselves up. So you need an ammo type without friendly fire (unless you want to turn off friendly fire on the vanilla ammo themselves, which I don't want to do).
Second, there's a thing with the spawn points of the bullet animations/projectiles. If you put a vanilla weapon on on a spidertron (that you made bigger/smaller/higher/lower) and shoot, it tends to look completely off (bullets spawn in mid air for example). You can modify the attributes of the gun the spidertron uses, but the ammo is shared with others, and both the gun and ammo properties are important for this. If you change the ammo prototype to have the projectiles spawn neatly on the spidertron, then they will no longer be aligned on the other uses (turrets, handhelds, and so on).
So, to make sure that everything looks good and can be easily balanced with other mods, I opted to have special variants of the ammo that I have full control over. I do think it makes for a little bloat, but it's the only way I've thought of to make sure the weapons look like they belong on the spider.
As for the actual weapon behavior, I guess this is personal preference. I play with the Distant Misfires mod which makes all weapons projectile based. I could try to have the option to choose between projectile and hitscan for the bullet weapons, but at the same time I really want to avoid introducing conditionals that require a lot of maintenance down the line (plus I would balance hitscan weapons differently).
Lastly, strange that Rocket technology would be a prerequisite for the basics. I'll test it with Space Age myself and see what's going on.
