Whoever told you it was a good idea for folks to come early to light candles and venerate the icons was indeed right. That said, it would be wrong to say that folks cannot do so after services start. There are however, certain times that it should not be done at all: at any time after the Liturgy of the Faithful has started and before that, during censing of the church, the Lesser Entrance, "Come let us worship and fall down before Christ," The Thrice Holy Hymn, reading of the Epistle and the Gospel, and the homily.
In my mind, the important thing to remember is that one can light candles, pray and venerate the icons at numerous times. On Sunday, one can do so before and during Hours/Matins, the Liturgy of the Catechumens (see exceptions above) and after the Post-Communion Prayers. Plenty of opportunities obviously; ergo, the advice given to you was right.