Hi Ed,
Well for Six-Lines your far better with a single as two will fight to the death. Six-Lines also like to eat small worms and crusteceans, so a healthy tank full of this type of life is desired. My Six-Line eats worms and cruteceans throughout the day, but it will also eat prepared foods as well.
Sandsifting gobbies are ideal for cleaning the bottom. My two Yellow Watchmen are great and they live in the same burrow. With bottom dwellers you could introduce them both at the same time, just be sure to get two different size fish and they should pair up nicely. I have had two pairs over the last six years and I find if you bring them home at the same time, even in the same bag they will pair up better. IMO I think they bond when they are both stressed in the bag on the ride home. There are alot of gobbies available, and I prefer the bottom dwellers to help out in the house cleaning department. As for Blennies, they are also a great fish, I would just never mix Blennies and Gobbies in the same tank, as they would most likely fight to the death also. My two Watchmens do sandsift all day, however they would never starve to death even if they wiped out the entire sandbed because I feed them twice a day with spirulina and brine. So alot of the time they will simply rake the top of the sand to collect settled food particles, while the sandbed remains healthy and alive. My watchmens are now two years old each, and the pair before them were both 4 years old. While yes, some gobbies are harder to keep such as Clown's and Mandarin's because of there feeding habit's, others such as Sleeper's, Watchmen and Neon's are easier to keep. I know quite a few people who have kept Mandarins and Clowns living in there reefs for years as there tanks are mature and full of worms and crusteceans. I find the burrowing variety's easier to keep because they will eat prepared foods as well as sifting the sand. I did have a Catalina goby for almost a year, up until the last heat wave of the summer when my reef reached 86* for a few days, which for this California cold water species was simply too much heat to handle. So if you decide to go with a goby or two, do your research and make sure you can provide the ideal conditions.
Cheers, Rich