Do a search on “sink tips” on here and you will find all the info you can stomach.
No, the rating for a skagit-style head is NOT for the head plus tip, just the head. There are a very few companies like Meiser that have an “advanced” labeling type that includes the total weight, but if there is only one number on the rod, or a labeled range of say 100 grains or less then you can be sure it is for the heads only.
The matching of the tip to the head, or a tip/leader of any kind to any kind of line is based largely on the linear density (grains per foot) at the connection point. This should go down as you go from head to tip, but they should be close without getting too close. It is THAT which is responsible for the smooth transfer of energy more than the total weight. This matchup is much more important for tapered lines with lighter ends, and one might say that the whole point of a skagit head is that you can get away with a wide range of tips. But the maximum density that can be carried by any line is more or less set by the density of the front of the head, but since “standard” skagit heads are all close to level section of line of approximately the same length this maximum carrying capacity goes proportional to the total grains of the head.
The more skilled you are the more you can explore pressing the limits but a good rule of thumb for me is that it is “easy” to cast up to 15’ of t11/t14/t17 on a skagit head matched to a full length 7/8/9 wt (NA style ratings) spey rod. So for example if I know I will need to cast t14 for an extended period of time I try to bring an 8wt spey rod.
The total weight matters less. Within limits once you find what you like you can just scale the depth by scaling the length of the same material as many people do. As an extreme example if you like 10’ t11, for example, you will be able to cast 20’ (twice the tip weight) of the same. It will not be exactly the same experience of course, and harder partially because of the radical change in geometry, but it WILL work. In contrast if you try to cast 10’ of t22 (the same weight but twice as dense) you may find the experience very problematic.
Though there are limits, a skagit head is VERY forgiving - again one might say this is the whole point of skagit heads - and you can vary the density of the tips as well. Within limits of course - and far more than you could with a delicately tapered line. So for example you can vary the depth by using the same length of different materials like 12’ of t8/t11/t14.
Lastly if you are an Epicurean type you can use compensated tips like the Rio
replacement tips. They tend to be more expensive but different sink rates in a given set have the SAME linear density AND weight, but different diameters. That way you can change depth while keeping the casting experience (both linear density and length, so here also total weight) the same for all the tips. That is far more important for tapered heads with lighter ends, but works with skagit heads as well. But if you use this method you will literally not be able to notice any difference when casting different sink rates. Beyond the fact that your anchor will sink differently - there is no getting around that.
I hope this helped a very little.