I personally would not use a forum for the front page of a hosting company. If I am looking to purchase any hosting services, I want to see what the company is all about and, most importantly, their plans and pricing. Definitely not a forum.
If your looking for a hosting company, or to be fair any company, I'd certainly not use a forum for it's home screen. The first impressions of any page, moreso for a company who wish to sell you products, are very important and you want access to key information to be easy for potential customers to access thus making a forum not the best choice for your homepage 😛
I agree with the people above. If you are going to be a hosting company then that should be your primary concern, it should be front and centre, while your forum comes secondary.
It should definitely be secondary. If you want though, just have the main URL re-direct to the forums that way you don't have to move your files if you change your mind later.
I would definitely make it a sub-domain. If I was looking for hosting and came across a host that used a forum as their homepage, I wouldn't take them seriously.
If you're selling something you want the product front and centre. Not to mention your forums will likely be a support section and you definitely don't want your visitors seeing your bug tickets before anything else.