Ok - yes - My reasoning would be tank glass integrity - IF you have glass of say e.g. 12mm - then by all means you can drill many holes in it - BUT, for a tank this size you are MOST likely going to go for e.g. 6 or maximum 8mm glass, right? Too many holes will decrease the strength of the glass, which could MOST likely cause the tank to crack (the bottom pane in any case)... Causing a HUGE water spill in your living area....
Regarding your second question - YES - it should be big enough to handle the water coming from the main display tank during a power-failure, BUT, this will also depend on the level your over-flow sits at - ie. how much water can flow over your over-flow when there's no water being pumped back into the tank....
The siphon hole MUST be in your return pipe (another reason for having the return pipe return at the TOP of the tank, and not the bottom), otherwise, when the power goes out, ALL the water could be siphoned out of the tank, by means of vacuum, through the return pump....
The best would be to have a return pump in the region of +-2000 to 3000 litres per hour. You don't want the water to flow TOO fast through your sump... Otherwise the filtration does not work as it should....