Thanks everyone. :P I didn't want to waste my money on Cannonballs and that stopped me from using them even if they were quicker. It feels great wearing my Slayer Cape after spending months of work to achieve it. :) I pushed for around 100k Slayer XP a day. I would play from midday until like 4am. I wasn't really fast at Slaying sometimes but I got what I wanted in the end.
I got this with just 5 minutes until midnight last night. :P I should have Level 99 Slayer by the end of January if Summoning or real life doesn't distract me too much.
If this works out right, it looks like I've have Level 138 Combat with only Level 95 Summoning. Although I would probably want to get Level 99 Summoning to go with all my other Level 99 Combat Skills. Good work. :)
I think Rares will rise slowly because people will want to work towards them as an Achievement Item. Remember they're limited in quantity hence being called Rares. It may take time for Rares to rise again if Summoning is going to be costy because people usually want to waste all their money and get a High Level in a new Skill.
Unbuyable Skills because then rich people can't just buy themselves the Skill Cape. I would say Slayer, Agility, and probably Farming. I know you can buy Seeds for Farming but it's still very slow.