Let me help..
1. I have been riding rental boards and I know they differ but generally do they tend to be towards the soft or stiff end of flex? Depends where you get them. I'd say rental boards are most likely going to medium flex to accommodate riders of varying skill levels.
2. I normally ride a 155 board...I am happy with this size but from what I have read it seems a little small for my height/weight. Any advice on whether to go with a longer board? For your height, a 155 is probably fine for the type of riding you're doing. You could size up if you wanted something a little bigger. 158 would be fine. I would not size down, unless you're trying to ride park A LOT and want a deck that can jib and spin easily. I'm 5'10" and ride a 156 most days.
3. I have seen a board that is a DC flex rating of 7...will this be too stiff for my height/weight? Would you recommend a softer board for someone who has never own a board before? Flex ratings vary between companies, so don't take DC's rating system as final. If its a 7 (10 being very stiff, 1 being very soft) then you should be fine. If you're an intermediate rider just looking to ride and not necessarily "specialize" in park, big mountain, pipe, etc, the mid-stiff flex rating will probably work well for you. Regardless of what board you get, you will get used to the flex and it won't detract from your riding that much, if at all.
4. Any advice on buying a DC Tone 2013, DC Ply 2013, Burton Process Flying V and Burton Blunt? Would one in particular and if so why? I am particularly interested in finding out about the DC tone as there is not much info out there about it. I don't know much about these boards and can't speak to them.
5. I am also considering a Burton Clash but am worried that I may out grow it quickly and as it seems to be aimed at beginners I am concerned that I may not progress on this as much as the others? Again, I do not know much about the Clash but I think you're right and it is a board more suited for beginners. It doesn't mean you can't ride it though. Progression is not dependent on the board you have.
7. Any recommendations of other makes of boards would be welcome too Check out the GNU Carbon Credit. Its a solid mid-level board thats not too expensive that will be a deck that you can use for a few years. My friend has one and loves it - its a "do anything" board. Hope this helps.