Since I do not believe this is consistent with mixing Standards, I would not recommend.  If you must try, then mix all at 5000 mg/l (ppm) each as your concentrated stock, then combine 200 ml of each concentrate to get one liter of mix to result in 1000 mg/l of each.  This may work as long as the chemicals of interest are not duplicated.  I cannot guarantee there will not be chemical interactions, precipitates, etc.  It it works, you will save alot on glassware and perhaps time, storage space.  Please wear goggles and appropriate protective gear in a fume hood, just in case, when mixing.  I would validate results with uncombined standards. 

We use a similar Standard in soil analysis. 

K = 1,907 g KCl Potassium cloride per liter (1000 mg per L)

Ca = 2,497 g CaCO3 Calcium carbonate (Needs 10mL of HCl for total dissolution) per liter

Mg = 1,0000 g of metalic Mg per liter (Needs 20 mL of HNO3 for dissolution) Mg salts are usually too hygroscopic too be used as standards

NH4 = 4,716 g of Ammonium sulphate

Fe = 1g Fe metalic per liter (Requires 30 mL of HCl 1+1 for dissolution)

Prepare a stock solution of each standard (more concentrate) than combine them to obtain the 1000 mg / L standard.

Standard means high degree of accuracy. Usually our observation based on reliability of standard solution. It is difficult job ( time consuming and castly  ) to make combined standard solutions after using different chemical, weighing, dissolving and other treatment proceeds. Some error beyond our control also. It is difficult to know the accuracy also. Presently combined standard solutions are available in market. Such are reliable, economical and may save your time also.