I have a good domain but everything is taken that is reasonable sensible involving my two main keywords? is the domain keyword1keyword2online.com a good alternative ? will the word online take away from my two other keywords? or is there something better like keyword1keyword2inc.com?
Why not? If your domain name remain short I suppose that is very good idea. However using your business name for the domain name can be good solution as well.
I think it will be fine to use 'online' in your domain, you could potentially make a really good name by using it, Good luck. Tell us what domain you choose in the end.