S/PDIF and TOSLINK are limited to 2ch PCM or 6ch Compressed DD/DTS, so while they CAN and DO work for 7.1 it's going to be up to the receiver and DPLIIz or equivalent to add the two extra channels (which would be SB, FH, or FW; can even do 9.2 w SB+FH/FW). It's not ideal, though.
HDMI supports ARC so long as your TV does, which means an HDMI cable run from the TV to receiver could send the exact audio data from thetele vision to be output by the receiver, including DTS-Master Audio and the more common but (in my opinion inferior) Dolby True-HD "lossless" audio as found on Blu-ray tracks. This will give you native 7.1 assuming that the source is 7.1, which is often not the case with PC games, which are usually 5.1 and even those that say 7.1 are just simulating the 2 extra channels. This may have changed or it may be that the games I have are completely different from you, but I would not expect to have true 7.1 with everything and then the only guarantee is with Blu-ray discs that state the type of audio used.
This said, I have a system that's set up as 11.2ch, which is the 2 surround rear channels that 7.1 provides along with two front height channels and two front wide channels, along with a second subwoofer. As this is not an official surround configuration, almost everything has to have the FH+FW channels "simulated", although a good receiver will do a very good job of this. The purpose offfront height is to give the sound a vertical field, and you can hear things flying overhead and they actuallysound like they're above you. The front wide channels are designed to bridge the gap between the front LCR speakers and the primary surround channels, which are to be directly beside (no more than 15deg behind) the listener and ideally about 3ft above the ears, which is the llargest gap in a system. The sound field is enhanced and positional or directional audio greatly improved by their addition. The second subwoofer is necessary for large rooms, but also boosts bass without making it boomy, and further works to decrease any localization of the source (you should never be able to tell where bass is coming from). I have had success with the two subs in either front corner (one per corner) as well as having them slightly in front of the listening position off to the sides, having one in the front towards either side and the other in the rear towards theoopposite side, and so forth. It depends on the room acoustics. I will say that I thought 2 subs was pointless until I added the second, which is the same 12" 500W powered sub as the first, and the boominess, bass reflection, and such disappeared while the (apparent, not measured) frequency response improved so that where 20hz had previously sounded very strained and unnatural, it now had a perfectflat even sound due to cancellation and other acoustical effects.
Also, one of the best things you can do for the sound of your system is to not use a center channel speaker (ie horizontal WTW/WWTWW/WTWTWdesign) but instead get a third matching tower to the front left and right speakers. If impossible, get the largest "bookshelf" version of the tower and use it in the proper vertical position. The difference between a horizontal center and a matching tower is immense, completely changing the system for the better, and a bookshelf while not as perfect is still a big improvemen.
FFurthermore, if you can use the same towers for surround, at least the actual surround and not rear surround, everything will sound far better, richer, and more natural as you won't have dinky 4" "woofers" as the largest source of midbass outside the LCR "plane". While the same towers for SB is even better, the SB speakers are not utilized for much more than ambient noise and the very occasional directional sound (SR/SL are the directional surround), and I have found that speakers designed for a very wide dispersion such as dipole speakers (Def Tech has very good ones) are an extremely close second, and they have an immense impact on ambient sounds, making them natural, non-directional, and essentially adding a whole bunch more layers to the sound field while not making themselves noticeable. They are the best thing you can do to boost immersion. The difference between using dipole speakers (DT/KEF or for less expensive but not as good, yet still better than any bookshelf, Klipsch) is extremely difficult to put into words, but extremely apparent. I highly recommend their addition, and the current bookshelf speakers can be used for front high speakers (front wide benefits from dipole a huge amount, but front high can do 90pct with bookshelf speakers, while FW/SB are going to only give maybe 30pct of what they can, obviously making them priority over FH).
If you add a second sub, remember that unless it's the same exact sub as what you have it won't be an improvement, the onlyeexception being if it's the exact sub just a different size (max +1/-1). If same sized, placement is universal. If different sized, I have yet toffind anything that works aside from one in front and one in rear, otherwise you'll get odd cancelation of frequencies and boosting of others, so say 20hz may seem more than 2x as loud, but 40hz is inaudible.
That's just a bit of advice from someone who has spent countless frustrated hours learning these things the hard way.