Rules are more guidelines. You'll find that you'll get a lot of information because different things work for different families.
The official safety guidelines are just about avoiding risk of SIDS which means you need to use something that's actually designed for babies to sleep in - not a car seat or a sofa or an adult bed or arrangement of cushions etc. Don't put anything in the cot that they could suffocate with, and don't overheat them with bedding, clothing, heating etc.
For the rest of the rules, they are all ideas of what is best practice - so it's best practice to keep the room cool, to have them near you, to have them sleeping on their backs etc but you also need to apply common sense and understand that you can't get everything perfect all the time and that's OK. So it's fine for you to pootle about in and out of the room they are sleeping in and it's OK if it's the middle of summer and the room is as cool as you can make it, etc.
WRT co-sleeping - the issue is that it's just not as simple as putting them in bed with you and everyone being happy. If you decide to bedshare you can do this safely but it takes some preparation - you need to remove pillows and duvets from the area where the baby sleeps and make sure the mattress is firm and the sheets well fitted, not loose. If you're breastfeeding you'll tend to naturally find that you curve your arms and one leg around the baby so that they don't wriggle up or down to where the pillows or duvet are. And of course if you co-sleep you need to make sure they can't roll out of your bed, which is where a lot of people find that a sidecar cot is a useful idea. (I'd personally recommend this approach very highly.)
Healthcare professionals blanket recommend against co-sleeping because there's too much information attached to "Oh but it's only safe if you do X and Y and don't do Z" - it's easier and simpler and less likely to be misunderstood to say "Don't do it". Unsafe or unprepared bedsharing is very risky.
(The don'ts are - don't co sleep if you or your partner smoke or if you've drunk alcohol or taken any medication which makes you drowsy.)
Personally I had a sidecar cot (I converted a normal one) for night time and in the day DS slept in a baby bouncer in the living room, or in his pram.