Some rambles from my own experiences...
Be aware that using an Interrail pass on local lines such as the FEVE and Cercanias networks away from Madrid can range from being a minor faff to an absolute headache.
Stations are mostly gated and unstaffed so you end up having to climb under/over them and where they're staffed, staff will usually only issue a gate pass for one journey at a time. A lot of them don't seem to like Interrail passes at all. However once you're in, particularly on the network around Gijon/Oviedo/Bilbao, it's pretty easy to just potter around for a few hours. I recall in the case of Bilbao, the woman in the ticket office just gave me a pile of gate passes without prompting- she at least was on the ball!
The only network I can say that was easy to use was the Cercanias Madrid network. Staff were always on hand (and a LOT friendlier) to let me out, though there is a local ticket for Madrid that covers all Metro/Bus/Cercanias anyway.
In essence, I'd recommend a RENFE Pass to link up the cities then just use cheap local tickets in their environs. FEVE tickets are very cheap indeed, though journeys can be very slow and a little uncomfortable (particularly along the north coast).
High Speed lines don't generally replicate conventional routes, but if you fancy a taste of both, some AVE trains are dual gauge (and go through a gauge changing machine part way through the journey). This is the case with Madrid to Gijon and a couple of other north coast destinations.
Conventional routes however seem to be very poorly served with frequencies being more like 1-2 trains per day each way out in the sticks. They're also often VERY slow- Madrid to Valencia is possible with a regional service but it'll take you most of the day!
The only fully conventional route I've used is Malaga to Seville via Bobadilla. It's a very pretty run, too. You can then return on the "Avant" High Speed service via Cordoba.
As for photography, I've never had a problem and I'm sure that the previous rules about permits were lifted a few years back. The only network I've seen with a no-photography rule was the Seville Metro- though I got away with taking a few snaps as I pleased. However, be aware that in Madrid there is a massive push on preventing trespassing and security guards roam the network constantly. I was once chased because I was sauntering to the end of the platform to take a photo of the train I'd just arrived on- thankfully it was taken in good humour (and a chorus of "ohhhhhh") when I spun around and pointed my camera at the train
