Bike Forums - Hardrock suspention adjustment

Hello I have just joined this forum, so my apologies if this has already been asked, I couldn't find it anywhere.

I have a 2011 Hard rock which I used for about a year as up until a few weeks ago, I haven't touched it. In the past few weeks I have been using it for road riding. Now before I get told this is a dumb thing to do, I just want to say I wont be buying a new bike anytime soon.

I have tried to make the suspension as "hard" as I can but I seem to just be able to spin the "preload" to many times. When I turn the preload up I think it does make it slightly stiffer (I have tested) but there seems to be a noise like the sound of a spring, this is why I stopped, will the preload eventually stop turning and reach its max or should I stop Turing it because it has reached its max?

I have done the obvious such as make the tires as hard as I can (60 psi), get slicker tires... which leads me to my next question. I am fairly sure but not 100% that the inside rim width is 25mm, through some graphs I have found the minimum tire width I can get is 1.7 - 1.75. Is this true, I have found some 1.65 slicks that i really like, is it dangerous to run these?

Thanks for your time.