Betta Companions

My boy, Finnegin, has been living alone since I got him in August. I've been toying with the idea of getting him a companion or two, but I'm not sure. My concerns are:
1. Is he really bored and in need of a fish companion? (He's in a 10 gal. tank w/lots of plants and caves, but I'm at work all day, so he only gets attenion from me @ nights and weekends).

2. What type of fish would be good? I've heard tetras or neons. My concern there is that they eat diff. food (flakes) which I've heard make the water dirtier.

3. My third concern in introducing new fish to the tank is that I wouldn't be sure of the health of the new fish and wouldn't want to introduce some sickness they're carrying into Finnegin's home.

Is it cruel to just leave him alone?

I think bettas can and do get bored if not given enough stimulation. You might consider getting a mesh divider for your tank and getting another betta for the other side. I have two divided tanks and the bettas seem to enjoy watching each other. They flare at each other every once in a while but nothing excessive. Without a fish companion or at least something like a snail (if the fish is docile enough to accept one), they have nothing to entertain them if you're gone all day. That can lead to neurotic bad habits like tail biting and pacing. Another plus to this approach is that you can enjoy the beauty of two bettas at the same time rather than one.

My Triton started tail biting because I'm out in the day and he was bored alone. After a lot of research I added 6 pygmy cories and now he's apparently enjoying their company, enjoys watching them and his tail is growing back nicely!
All just depends on the individual betta really, it's always a risk adding new fish but after some initial chasing and nipping my boy gets along with them just fine - he's the king of the tank

As an addendum, I originally had my pink veiltail Lotus in a 6 gallon tank by himself. It was heated, filtered, fully planted, lots of hiding areas, but he started pacing like crazy. He did not look like a happy fish at all. But once I re-arranged the tank, put a divider in, and added my delta tail Sequin in the other side, Lotus is a different fish. He is much more relaxed and just swims around majestically. Normally I would want at least 5 gallons per fish (they both have 3 gallons now) but they both appear very content, so I'm just keeping them this way.

If it was me i would add some tank mates. 10G is a fairly big tank so you have a few things to choose from. I would go for
8 Pygmy corys
and
8 Ember tetra

They ember tetra will need feeding flakes or bits of crushed pellets. The corys will eat the same if it falls to the bottom but it would be best to get them some kind of sinking pellets.