Stockades work like minor depots, so if you have an unbroken line of stockades from Austin to El Paso, you should be okay, if you don't stuff El Paso with such a large force that they cannot be kept in supply, which really shouldn't be the case anyway. If your forces are that large, you're probably doing something wrong

.
IIRC rangers have a very high chance at successfully foraging. Foraging happens when a unit is low or out of supplies. It tries to find supplies in the region it is standing in or moving through--the more movement, the greater the chance of success.
Besides, Rangers are very small units and use little supply. They carry 4 GS and use 1 per turn.
Successfully foraging will give the unit some supplies; failing to do so will mark the region with a pillage icon, and that region cannot be foraged again until the next harvest season.
Generally, if Indians are attacking, I believe it is because the North has used RGD's (Regional Decisions) to "hire" the Indians as mercenaries. I don't recall "peaceful" Indians going on the war path and attacking Confederate stockades.
Supplies move automatically at the start of each turn before anything else really happens; absolutely before anything you have ordered to be done takes affect.
To get a full understanding of how supplies work you should read through the old AACW (AGEod's American Civil War) (precursor to CW2) forum article
Supply Primer.
Generally speaking supplies move, from one supply source to the next, by being pulled. The initial pullers are stacks on the map, especially if they have one or more supply units. Supply sources are primarily Depots, Forts (which includes Stockades and Redoubts) large cites, and cities with harbors, with the strength of the pull generated being more-or-less in that order, with combinations being cumulative. Stacks with supply units can pull from nearby supply sources (two regions at the most). Stacks without supply units in the same region as a supply source will take supplies directly from only it. A supply source having supplies pulled or taken directly from it then pulls supplies from another nearby supply source (5 regions away at the most), thus creating a chain of pulls.
Supplies are moved more-or-less as if a supply wagon were moving, but in three phases at the start of each turn, as if the hypothetical-supply-unit could move three times, once per phase. Since generally cities with harbors and depots generate more pull than cities with just depots, and a supply unit using the Riverine Transport Pool can move further from harbor to harbor than from city to city by roads, or by rail when using the Railroad Transport Pool, more supplies will be moved along rivers than rail lines or roads alone.
In all cases, you always need at least 25% MC (Military Control) in a region to move supplies through it, and any unopposed enemy combat unit will block your supplies from moving through that units region. Rivers don't have MC, but supplies can be blocked from moving through river regions by artillery being able to bombard into said river region (by being good-order and at least at entrancement level 3 or more, or in a Fort, Stockade, or Redoubt.
Units lacking supplies, especially if they are moving, will take hits from Wear-n-Tear™

. Militia and infantry and cavalry in general, can only carry enough supplies to be away from a supply source for two turns, at the end of which they must be on a supply source again, or suffer the consequences. Supply units will lengthen this time considerably by being a mobile supply source.
You periodically get options to increase your RivTP (Riverine Transport Pool) and RailTP (Rail Transport Pool) under <F3> Department of War. For each 1/3 of full capacity (check the tool-tip of the Train and Steamboat on the top left of the map), one supply phase may use Riverine and/or Railroads to move supplies, as the case may be (it may use both in one phase, same as a unit may use both while moving); otherwise it will move only overland by roads and tracks.