Yes, you can do 7 workouts every 2 weeks instead of 3 per week.
Mark Rippetoe has stated that doing a Starting Strength workout every second day is fine for a novice, and that the normal 3-per-week scheduling was chosen because for the majority of people, it fits better with their work/life scheduling. I expect that Stronglifts is similar enough that the same advice would apply for that.
However you shouldn't be tempted to try to add any additional workouts beyond this. As a novice you should only need 48 hours rest to recover from each of your workouts, but that rest is still crucial, and should be considered (along with nutrition) as being equally important as the workouts.
I have a really hard time gaining weight
(as I said, extremely high metabolism)
No, you do not have a high metabolism. Your basal metabolic rate is determined by the amount of muscle mass you have, nothing else. If you've previously been able to eat whatever you want and still stay skinny without significant exercise, it's because you have a lower appetite than average, and "whatever you want" is just less food than what the average person likes to eat. You'll find that if you force yourself to eat more food, you will be able to gain weight.
I plan on getting a total of 3355kcal a day consisting of 252g protein, 130g fat, 293g carbs based on TDEE calculation when bulking. Maybe more than this if I don't gain any weight (as I said, extremely high metabolism).
This is an excellent idea, although I'd suggest that your planned protein intake is very high, which isn't harmful, but which may be a difficult target to reach without making protein supplements a major portion of your diet. Really, an upper limit of 2 grams of protein per day, per kilogram of bodyweight should be plenty.
You could also very likely start at a lower calorie target, say 2800-3000kcal, which would still be enough to make initial gains, while allowing you to more gradually adapt to the increased amount of food that you will need to eat. Then, if your progress stalls either in terms of weight gain or strength gains, just keep bumping your food intake up in increments of around 200kcal/day.
If you're not already planning this, a food intake tracking app if a very useful tool to ensure that you're eating enough.
This answer of mine to a similar question may also be useful to you.
I have heard a lot of people say that 5x5 isn't optimal for putting on size, would it be better to instead follow the same program but do 3x12?
Higher reps for hypertrophy is very much old-school bodybuilding thinking, and the science really doesn't support it. A 2014 study found that a higher intensity, lower rep (7x3) program produced the same increase in muscle size as a lower intensity, higher rep (3x10) program, while also resulting in significantly better gains in strength.
Additional reasons why you shouldn't do higher rep work as a novice:
- The 10-12 rep range associated with bodybuilding is used by trainees who are already very strong and are already so big that they have great difficulty adding more muscle. Higher reps might be useful for adding an extra 1kg of muscle to a bodybuilder who is already 120kg, but when you're a 61kg novice and looking to add more like 20-25kg of muscle, they are hugely suboptimal.
- Higher rep workouts require performing reps while fatigued, which will compromise your form. This is a very bad idea for a novice lifter.
