I do not see any reason why you could not use Solar oil to maintain Calgel. I have some solar oil which I use on my clients and I am a Calgel user.
Calgel would probably suggest it is best to keep to one product brand. This makes sense if it comes to mixing gels, L&P etc....
Any reputable cuticle oil would be fine. I'm also a fan of NSI Nurture Oil plus I have Calgel cuticle oil also.