Is there any rule against making my drummer and my bannerman the same model? and also if i have a unit of 20 saurus and if the above is true then 1 model has drums and a banner, does that negate their melee wpn attack? or do they get a spear/club attack and jaws as well ?
Rules as written no, but the rules are also wiziwig (what you see is what you get). So, you would have to model it, and be aware is is obviously subject to change.
In my signature is a link to my Insta with an example of this modeled on a knight I made with both drums and banners. it is technically better to do this (and within the rules) as it preserves both bonuses until the second to last model is slain provided you remove models the same way I do (regular models > drummer/banner > Alpha)