Linked here is an amazing article that I suggest you read from the Starting Strength wikia. I will highlight some of the key points below, as well as provide some of my own advice.
Muscle builds most rapidly when adequate supplies of micro- and macronutrients are available at all times. This rarely happens unless you are eating a caloric excess.
In other words, if you want to see optimal strength and muscle gains, you should be eating at 500 calories above your daily caloric maintenance level.
In order to burn bodyfat, you must take in fewer calories than you need. This generally will result in you taking in less micro- and macronutrients than you need to build muscle, even if you take every supplement on the market.
In contrast, if you wish to burn bodyfat, you will need to be running at a caloric deficit (generally speaking 3500 calories = 1 pound fat, so 500 calorie deficit / day is the target). You should already see the problem here: you will have to choose which is more important to you, gaining muscle or losing fat.
Ideally speaking, any initial weight training will be done with a minimum of cardio and while eating a caloric excess. This will allow for optimal muscle growth during the time in your training "career" that is optimal for that muscle growth. Less cardio = more calories for growth, hypothetically speaking.
I totally agree with the sentiment that if you are going to do SL/SS, you should commit yourself 100% to the program to get the most optimal muscle gains; even at the expense of gaining an extra couple pounds of fat. As a bonus, you being an endomorph means that you should see pretty awesome gains on this front.
Once you exhaust your beginner's gains and have completed the SS/SL program, you can then shift your focus to losing your body fat while maintaining your strength and muscle. For this, I would recommend a three times a week HIIT cardio program, supplemented with a once or twice a week lifting routine to maintain your muscles and lifts.
Also, you will want to ensure you measure your progress using both weight AND body fat %. Knowing both will allow you to determine how much body fat you are gaining / losing with respect to your muscle, and provides much needed context to your progress for both goals.
Endomorphs should rest less between sets in order to thrash the metabolism
I don't recommend this at all. Do not rush, but instead take your time, get adequate rest, and always focus on your form. Whatever metabolism gain you get from this practice would be insignificant compared to the loss of muscle gain from not resting, or worse yet, the risk of injury from not resting enough. This is especially true when you get to the higher weights at the end of the program. EDIT: to clarify, I mean as part of the actual SS/SL program. Once you have exhausted your progressive gains from the program and have shifted focus to cardio and fat-burning then I would suggest giving this a shot.
It goes without saying that diet is imperative. I think my diet is actually okay, I think I just need to halve my meals and eat the same amount over 6 instead of 3 and cut out the odd sugary snack I fall prey to.
Definitely cut the sugary snacks, and focus on getting a well-rounded diet established. The linked article at the top goes into explicit detail on this subject and has a lot of great advice. Once again, I suggest you commit with the 500 excess calories/day to optimize your gains for the program, and then shift your diet afterwards to be more focused on fat loss (500 day deficit + cardio).