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Normally you can change textures re scale etc... unless the artists states that you cannot at time of purchase, normally the only rules are,
you cannot sell the models, you cannot post them on any other site, they are only to be included in any game you make.
as far as models downloaded from the GG forums they should not need importing 99% of them are ready to go in GG, if you setup folders in your:
gameguru\files\entitybank\user\
{NAME OF ARTIST}\{MODEL NAME} it will make life much easier for you within the editor.
don't forget scripts go into \scriptbank audio goes into \audiobank
If you are getting assets from 3rd party sites just be careful of the polygon/triangle counts it is best to not go above 20k the lower the better, now the current importer in GG is being updated/fixed until this is complete you need to import manually if you do not know how to do this send me one of the models and I will create a video outlining the steps.
**Important** make sure ANY third party model as commercial rights before using read the small print
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