With a 150w HPS and since you are obviously new to growing, i would start with 1-2 plants. You are not going to get a massive light coverage with that and you don't want to try and cram a load of plants in there and let them all suffer.
Also don't use anything other than Mylar for reflective material. Something that is not designed for light reflection (what you have is designed for heat/IR reflection which is very different) as this will cause hot spots. If you cannot afford or cannot find Mylar sheeting, just use white paint. For the scale you want to start with, it wont matter too much. You just want to get the basics of growing first. You don't learn to drive in a Lambo so you don't need a super setup for your first grow.
Don't use dixie cups, use actual plant pots with the drainage holes already in them, they are super cheap. The growth height is not dictated by the pot size per se, it is mostly the strain and conditions the plant is growing in. I have had plants veg way too long and when i put them into flower, they turned into monsters that took over my tent. Second grow, i flowered much earlier and they are 2/3 the size and will probably yield more as i have learned what to do, and most importantly, what NOT to do.
You can read the entire internet on growing, but until you get that first plant from seed to smoke, you really don't know anything. You need to learn from experience. Don't over complicate it, grow in soil if you can, or grow in coco if you are going hydro, DO NOT use aeroponics, clay pebbles, DWC or any sort of feed direct to the roots system. These systems, while awesome, are for advanced growers and allow very very little room for error. Soil and coco acts as a nice buffer between when you fuck up and when that fuckup will kill your plants.
I would suggest getting 5 feminized seeds from a seed bank, don't do all this paper towel crap you read about, it is pointless, you just end up handling a sensitive seed and root in it's most delicate stages. Just pop the thing in a peat pellet and cover with about 5mm (twice the length of the seed itself) of soil. Don't leave it too shallow or it will push itself out but definitely don't bury it too deep as it will never make it out of the ground. Keep it moist, but not wet. A good tip is to very gently squeeze the peat pellet. If water pours out, it is too wet. If you see water form at the bottom of the pellet but it doesn't fall, you are good. Practice with a pellet without the seed first so this makes sense to you.
Out of these 5 seeds, you should get at least 2 pop. They may pop in 2 days, they may not pop for 2 weeks, it just depends. Don't panic, just check how moist the pellet is each day, don't let it dry out, and you will see them pop soon enough. If after 21 days you see nothing, it isn't going to happen, so break the pellet open and see what went wrong. You will learn either you did something wrong like bury it too deep, or maybe the seed was just a dud. Either way, you learn.
Once you see a tap root come out of the pellet, it is time to pot them up. use a 4" pot with whatever medium you are going to use. If the seedling is stretched out, bury the pellet deep into the pot so the stem is about an inch from the soil to the pre-leaves. Sometimes seedlings go mental when they pop and get too tall for their own good. This is how you correct that.
Remember, seedlings need humidity. It MUST be over 75%, so get a heated propagator if you can, if not, just a standard one, and a mist bottle. Don't mist the plants themselves, just the inside of the lid of the propagator. A dry seedling is a dead seedling.
Once you have them growing in a pot, wait until they are about 2-3 inches tall and have 6 leaves. Then pick the two largest and healthiest and looking plants and trash the rest. Brutal, i know, but focus on the best of the bunch and you will be just fine.
I won't go into details of what to do after this, because it is a lot to remember. Just get your seedlings strong and then you will start to get the idea of what growing is all about. Getting strong seedlings i think is hardest part. Once you have this down, come back to the forums and see what to do next.
There is nothing to stop you popping open the pellet after a couple of weeks to find out why it hasn't popped if nothing happens, so i don't see the difference really. Moist napkin, moist dirt. I just dont have to fuss with tweezers getting the cracked seed in a pellet root down afterwards. I find popping seeds the natural way happens within 5 days on average. couple of days difference means absolutely nothing to me. There is no rush.