This is an excellent issue and I am kind of currently going through with my newly opened poolhall. We currently have 5 7 footers, 4 9 footers and 4 12 foot snooker tables. It all depends on the market you serve and whether or not there is a market to begin with for snooker. Currently. there's only 1 poolhall in southern California other than my place that has snooker tables. If there is demand for it, I would definitely introduce snooker since the rate is higher than traditional pool. But table cost, maintenance etc. also goes into consideration.
When we first opened up, the idea of the 7 footers were for APA matches. We've been opened for 2 months and we've had more nights than we can count that customers are waiting for 9 footers. We usually have them start off at 7 footers and when a 9 footer opens up we switch them. However, we are probably looking to add in a 9 footer to meet demand.
If this is a brand new poolhall that just opened, I would wait to see what the market demand is first before making any serious changes, both financially and time wise. Your customers should be the #1 indicator for making a big change like layout and tables within the hall.