All Bittorrent programs have peer connection limits. Also trackers don't give your program all peer's addresses in one response, usually there is a limit: about 50-200 IPs. Basically you don't have to connect to everyone in order to download fast. More connections mean more resources consumed, both PC and network resources. From my observations on a few years old machine, processor and memory are enough to run many (a few dozens) active torrents in Tixati. If system becomes slow and unresponsive, it is usually a sign that the hard disk drive is struggling with too many tasks simultaneously.
What tells you that there are more seeds and peers available? Torrents sites take this information from a tracker, numbers reported by public trackers are almost always not correct. Other Bittorrent programs aren't a good source of information too. Someone told me once that he prefers uTorrent to Tixati, because on uTorrent he sees more seeds and peers available than using Tixati. This is because Tixati reports seeds/peers numbers correctly and uTorrent guesses or makes up. You can verify numbers you see in Tixati, all those addresses are on the peers list. uTorrent hides addresses of unconnected peers by default, but even if you enable the option to display all addresses, you'll see that there are far less IPs on the list than uTorrent displays in seeds and peers columns (for torrents with very high peers numbers of course). It seems that uTorrent picks the highest peers number from all trackers, then adds peers obtained through DHT and PEX, which are most likely the same addresses as from the tracker (basically uT is counting some peers twice), and even then these combined number is often lower than number displayed in peers column. I don't know from where it gets the remaining difference. I wouldn't trust uTorrent when it comes to available seeds and peers.
Concerning your second question, I think the best method is to use default settings, which work very well, and modify only those that depend on your Internet connection. For example you should set reasonable bandwidth limits and active torrent numbers. Also it is very good idea to configure your firewall and broadband router, so that Tixati can receive incoming connections. This will allow to connect the most peers. I think using both IPv4 and IPv6 protocols isn't necessary. Tixati can connect to almost all IPv6 peers using IPv4, because probably everyone that uses IPv6 also uses IPv4. I think you can spare some resources by using IPv4 only, but this is most likely a very minor gain. For details read the Optimize Tixati help guide:
http://www.tixati.com/optimize/
Oh, I almost forgot it, you should add Tixati to your anti virus program's exceptions list, this helps a lot too.
