Hi Jon. I have a few thoughts on your questions.
1) You can go with 3/4" gravel, but it is a bit of a crap shoot. You need to be very careful of the source and make sure there is no limestone in it or you will be battling to get control of your pH. Also pretty tough on your hands. A new alternative, expanded shale, is popping up more and more. Less expensive than Hydroton, U.S. mined (vs. Hydroton), lightweight, porous, and easy on the hands (it's been smoothed out a bit). A community member, Rob Torcellini, is launching a new "Rocks in a Box" expanded shale product on March 28 and we (The Aquaponic Source) will be his distributor - http://www.theaquaponicstore.com/Aquaponics-Expanded-Shale-Grow-Med...
2) Many people are exploring hybrid systems that use a media bed as a primary filter before the water goes into a raft or NFT system. Is that what you are referring to here? If so, you aren't alone! Here is a group in here dedicated to talking about this - http://aquaponicscommunity.com/group/Combination. You might need to do more filtration before going into the PVC, and watch out for oxygen levels in those pipes - you will get pythium (root rot) if you don't have enough oxygen.
3) Pumps etc. depend on a lot of things, mainly now far and how high you need that water to go. Need more detail
4) Just started carrying a new product a week ago that we are pretty excited about for this very issue! It is called a Water Alarm and sounds a loud alarm if it senses water. Also comes with additional sensor pads so you can string multiple sensor points to one alarm. Cool, eh?
1-gravel does work. As Sylvia said, you need to do some research into the gravel before you go getting a truck load. You would want a quartz type rock or some non industrial waste type lava rock can work but definitely hard on the finger nails. I've been using 1/2" brown river rock which in my location is mostly quartz type river pebbles. This works very well for beds with timed pumping to them but is a bit smaller than works best for a constant inflow where a bit larger like 3/4" would be ideal. Crushed granite or basalt of the right size would work too (it's what the aussies call blue metal but it isn't metal at all.)
As for pea gravel, that only describes the size/shape of the gravel, If I were to simply order pea gravel here, they would send limestone and that is not good for aquaponics. Pea gravel is also a bit small for a constant inflow set up using siphons. All of the above listed are quite heavy except for the lava rock. I haven't yet gotten to try the expanded shale but it sounds promising.
2-NO, do not pump fish tank water directly to plants growing in pipes, the fish poo will get all over the roots in the pipes and cause all sorts of problems, including nasty smells.
2a-I would recommend setting up the system in a CHIFT PIST fashion so that the pump lifts the water from a clean water sump up to your fish tank (and also your pipes which can then drain to the fish tank or grow beds or even back to the sump tank) and the fish tank flows to the grow beds which then drain to the sump tank allowing for a clean water sump from where you can pump the water to things like NFT pipes.
3-share more of the system layout and total size for more help on pump choices. We also need to know the amount of grow beds not just fish tank size.
4-The water alarm sounds like a good idea to me. They do make Pallet spill containment type containers but you are likely to spend quite a bit of money on it and that won't necessarily save you if the leak or disaster happens at a grow bed or some other part of the system that isn't placed in the spill containment basin.
Jon, in response to #2, I have a 7X4 foot rectangular fish tank. I installed open rain gutter (plastic) around the top inside perimeter. I put about an inch of gravel in this and use it to grow fast growing plants. Some of the water returning to the fish tank is diverted into this stream. Right now watercress is going crazy in it. I have just added some parsley which is also doing well. So far no problems with clogging up although if it does, the water simply overflows back into the fish tank. Last year I had tomatoes growing in it and they did real well but the roots did clog the stream.