Having force users serve the Hutt Cartel is problematic, IMO.
If they are light siders, they should be in opposition to pretty much everything the Hutt Cartel stands for.
If they are dark siders, why are they serving Hutts rather than overthrowing and/or usurping the power of their weaker masters?
I'm not saying it absolutely can't be explained, just that there seems to be some significant roadblocks to having adherents of either aspect of the force follow Hutts. I suppose you could make them 'gray' force users, but the whole concept of gray force users itself contradicts established lore somewhat.
Your thinking of things from a jedi/sith standpoint. What we know as "Lightside" is defined by the jedi same with the darkside. Its very similar to how society will dictate what is "beautiful" and what isn't.
So a force wielding mercenary may use powers that draw from the lightside of the force, but might not have a jedi like personality. We often get confused because we have tied lightside and darkside to personality types and to emotions, which is not accurate.